Game Asset Vertex Count/ Importing in Unity
There are a lot of ways to optimize your model. For example, you can model the single floor, and instance it in the unity. You can use baked normal maps instead of geometry. It's all about the level of details you need in your game. If it's far from the camera you can use a simple textured box. If you walk around but you have no helicopter, you can have detailed first two floors, and less detailed ones above. So it's all up to you. The question is more to do with unity than with blender, so maybe it's better to get some guidelines from the unity gamedevs about you very problem - the best way to build a city in unity.